Thirteen independent autosomal recessive mouse mutations are known that cause defects in three subcellular organelles: melanosomes, lysosomes, and platelet dense granules. Their phenotype includes dilution of coat color, defective lysosomal enzyme secretion, and prolonged bleeding time. These mouse mutations are models of similar heritable platelet storage pool deficiencies (SPD) in humans, including the Hermansky-Pudlak and Chediak-Higashi syndromes, that share the same triad of defective organelles. The plasma membrane of most vertebrate cells is associated with an interconnected multiprotein complex, the membrane skeleton. Two membrane skeleton proteins, Ank3 and 4.2, genetically co-localize with two of the mouse SPD mutations, mocha and pallid, respectively. Moreover, these same membrane skeleton proteins localize to melanosomes, platelets, and lysosomes. These data lead to the hypothesis that organelle membranes are associated with a membrane skeleton, and that defects in the organelle membrane skeleton cause SPD in human beings and in mice. Ank3 and protein 4.2 represent two components of this structure. In this research, Dr. Peters will identify additional components of the organelle membrane skeleton, establish their roles in the mouse SPD mutations, and continue the analysis of Ank3 and 4.2 in the subcellular organelles of both normal and mutant mice. The specific goals of the proposed research are to: (1) identify new members of the membrane skeleton of intracellular organelles by ligand screening of expression libraries to obtain CDNA clones encoding proteins which interact with Ank3 and 4.2; (2) characterize and genetically map candidate genes for the mouse SPD mutations; and (3) determine the specific Ank3 isoform present in organelles, its genomic sequence, and the function of protein 4.2 in organelles. Ultimately, this research will identify the genetic defects that cause SPD in humans, a prerequisite to the development of effective gene therapy interventions for their treatment and eventual cure.